Titanium is one of the family of metals that can be anodized such that the anodized dielectric films deposited will pass current readily only in one direction. Said family is commonly referred to as "valve metals." Unlike vacuum tubes and semiconductors, valve metals do not find common use for rectifying electricity. Rather, valve metals find common use in electrolytic capacitors, aluminum and tantalum being the most useful. Electrolytic capacitors are used to filter and smooth out ripple in electrical circuits.
Electrolytic capacitors achieve that ability by virtue of the dielectric properties of the anodized films that insulate the two electrodes. Dielectric films are described in part by their dielectric constant. The higher the dielectric constant the more electricity a capacitor can store and the more effective it will be at smoothing out ripple in an electrical circuit.
Dielectric strength is also important. Dielectric strength describes the field strength a dielectric film will withstand before breaking down with a catastrophic flow of current. Dielectric strength is usually given in volts per centimeter that the film will withstand without arcing over. In working capacitors, dielectric strength is very important because higher dielectric strength permits higher working voltage.
Film resistivity is a third feature of capacitor films that is important. Volume resistivity refers to the tendency for a given film to leak off its charge. The higher the volume resistivity, the better the capacitor. Electricity that leaks away is electricity that does not do the job required in an electronic or electrical device. Electricity that leaks away commonly shows up as heat, an unwanted side effect in itself, especially in solid state devices.
The trend in modern electronic design it toward miniaturization. Electrolytic capacitors are needed that are more efficient on both volume and weight bases.
This invention involves improving the dielectric constant, dielectric strength and volume resistivity.
Table 1 illustrates current handbook values for several materials. Literature sources: Handbook of Chemistry and Physics, 66th Ed., CRC Press, 1985-1986; Handbook of Chemistry, Lange, 13th Ed., McGraw Hill, 1985.
TABLE 1 ______________________________________ DIELECTRIC PROPERTIES OF CAPACITOR MATERIALS AT 23.degree. C. Dielectric Dielectric Volume Constant Strength Resistivity Material 10.sup.6 Hertz kv/cm OHM-cm ______________________________________ Aluminum 7.5-9 70-102 1010-1014 Oxide Tantalum 9-15 25-70 1012-1015 Pentoxide Titanium 85* 40-85 1013-1016 Dioxide 165* ______________________________________ *Perpendicular to c axis **Parallel to c axis
Sources vary significantly with regard to dielectric values reported, by as much as an order of magnitude. But there is consensus that titanium dioxide has superior dielectric constants and volume resistivities. Neither of these quantities is unique. They are best described as tensors which means that the measured values vary with the direction of measurement relative to the crystal axes of the substances measured.
Table 1 shows that titanium appears to be a superior metal from which to manufacture electrolytic (or other types of) capacitors. There are several reasons why titanium has not been used commercially for capacitor manufacture. Chief among them is that even under the best anodizing conditions resorted to date, anodized titanium films exhibit residual leak rakes at least one or two orders of magnitude higher than shown by anodized films of aluminum or tantalum.
Nominal 85 weight percent ortho-phosphoric acid in aprotic solvent is a superior solution in which to anodize titanium, its alloys and other metals. Anodized films created by this method represent significant improvements over the earlier art. It remains however to produce films with high dielectric properties and at the same time minimize or eliminate significant leakage. This invention is directed towards that problem.
Water is a polar compound and that is the principal reason why water is such a good solvent for so many things. It readily donates hydronium (proton bound to a water molecule) and hydroxyl ions when inorganic acids or bases are added. This is why water is classed here as a protic solvent to the extent that water is present in the solutions of the poor art, excess hydronium and hydroxyl ions will be present.
Aprotic solvents essentially provide solutions that are poor proton donors. This is in marked contrast with protic solutions of the same anodizing acid. The aprotic nature of a solvent is qualitatively indicated by the lack of reaction between a 5 volume percent solution of nominal 85 percent orthophosphoric acid in the aprotic solvent selected and granulated ammonium carbonate. Solutions of phosphoric acid in protic solvents vigorously evolve carbon dioxide gas upon the addition of ammonium carbonate.